1. Field of the Invention
The present invention relates to a multi-zoned catalytic trap for treating exhaust gas streams, especially those emanating from lean-burn engines, and to methods of making and using the same. More specifically, the present invention provides a multizoned catalytic trap which abates NOx, CO and hydrocarbons in the exhaust streams being treated at initial engine start-up conditions as well as at subsequent high engine temperature conditions and exhibits enhanced durability after aging at high temperature and lean operation conditions.
2. Related Art
Emission of nitrogen oxides (xe2x80x9cNOxxe2x80x9d) from lean-burn engines (described below) must be reduced in order to meet emission regulation standards. Conventional three-way conversion (xe2x80x9cTWCxe2x80x9d) automotive catalysts are suitable for abating NOx, carbon monoxide (xe2x80x9cCOxe2x80x9d) and hydrocarbon (xe2x80x9cHCxe2x80x9d) pollutants in the exhaust of engines operated at or near stoichiometric air/fuel conditions. The precise proportion of air to fuel that results in stoichiometric conditions varies with the relative proportions of carbon and hydrogen in the fuel. An air-to-fuel (xe2x80x9cA/Fxe2x80x9d) ratio of 14.65:1 (weight of air to weight of fuel) is the stoichiometric ratio corresponding to the combustion of a hydrocarbon fuel, such as gasoline, with an average formula CH1.88. The symbol X is thus used to represent the result of dividing a particular A/F ratio by the stoichiometric A/F ratio for a given fuel, so that xcex=1 is a stoichiometric mixture, xcex greater than 1 is a fuel-lean mixture and xcex is a fuel-rich mixture.
Engines, especially gasoline-fueled engines to be used for passenger automobiles and the like, are being designed to operate under lean conditions as a fuel economy measure. Such future engines are referred to as xe2x80x9clean-burn engines.xe2x80x9d That is, the ratio of air to fuel in the combustion mixtures supplied to such engines is maintained considerably above the stoichiometric ratio (e.g., at an air-to-fuel weight ratio of 18:1) so that the resulting exhaust gases are xe2x80x9clean,xe2x80x9d i.e., the exhaust gases are relatively high in oxygen content. Although lean-burn engines provide enhanced fuel economy, they have the disadvantage that conventional TWC catalysts are not effective for reducing NOx emissions from such engines because of excessive oxygen in the exhaust. The prior art discloses attempts to overcome this problem by operating lean-burn engines with brief periods of fuel-rich operation (engines which operate in this fashion are sometimes referred to as xe2x80x9cpartial lean-burn engines.xe2x80x9d) It is known to treat the exhaust of such engines with a catalyst/NOx sorbent which stores NOx during periods of lean (oxygen-rich) operation, and releases the stored NOx during the rich (fuel-rich) periods of operation. During periods of rich operation, the catalyst component of the catalyst/NOx sorbent promotes the reduction of NOx to nitrogen by reaction of NOx (including NOx released from the NOx sorbent) with HC, CO and/or hydrogen present in the exhaust.
The use of NOx storage (sorbent) components including alkaline earth metal oxides, such as oxides of Ca, Sr and Ba, alkali metal oxides such as oxides of K, Na, Li and Cs, and rare earth metal oxides such as oxides of Ce, La, Pr and Nd in combination with precious metal catalysts such as platinum dispersed on an alumina support, is known, as shown for example, at column 4, lines 19-25, of U.S. Pat. No. 5,473,887 of S. Takeshima et al., issued on Dec. 12, 1995. At column 4, lines 53-57, an exemplary composition is described as containing barium (an alkaline earth metal) and a platinum catalyst. The publication Environmental Catalysts For A Better World And Life, Proceedings of the 1st World Congress at Pisa, Italy, May 1-5, 1995, published by the Societa Chimica Italiana of Rome, Italy has, at pages 4548 of the publication, an article entitled xe2x80x9cThe New Concept 3-Way Catalyst For Automotive Lean-Burn Engine Storage and Reduction Catalyst,xe2x80x9d by Takahashi et al. (below referred to as xe2x80x9cthe Takahashi et al. Paperxe2x80x9d). This article discloses the preparation of catalysts of the type described in the aforementioned Takeshima et al. U.S. Pat. No. 5,473,887 and using these catalysts for NOx purification of actual and simulated exhaust gases alternately under oxidizing (lean) and reducing (rich or stoichiometric) conditions. The conclusion is drawn in the last sentence on page 46 that NOx was stored in the catalyst under oxidizing conditions and that the stored NOx was then reduced to nitrogen under stoichiometric and reducing conditions. A similar but more detailed discussion is contained in SAE Paper 950809 published by the Society of Automotive Engineers, Inc., Warrendale, Pa., and entitled Development of new Concept Three- Way Catalyst for Automotive Lean-Burn Engines, by Naoto Miyoshi et al., was delivered at the International Congress and Exposition, Detroit, Mich., Feb. 27-Mar. 2, 1995.
U.S. Pat. No. 4,742,038, xe2x80x9cMonolithic Catalyst Support and Catalyst Deposited on the Support,xe2x80x9d issued May 3, 1988 to S. Matsumoto, discloses a metal substrate for carrying a catalytic material useful for the treatment of exhaust gases from internal combustion engines. U.S. Pat. No. 5,874,057, xe2x80x9cLean NOx Catalyst/Trap Method,xe2x80x9d issued on Feb. 23, 1999 to M. Deeba et al., discloses a method of NOx abatement utilizing a composition comprising a NOx abatement catalyst comprising platinum and, optionally, at least one other platinum group metal catalyst which is kept segregated from a NOx sorbent material. The NOx sorbent material may be one or more of oxides, carbonates, hydroxides and mixed oxides of one or more of lithium, sodium, potassium, rubidium, osmium, magnesium, calcium, strontium and barium.
Prior art catalysts as described above have a problem in practical application, particularly when the catalysts are aged by exposure to high temperatures and lean operating conditions, because after such exposure, such catalysts show a marked decrease in catalytic activity for NOx reduction, particularly at low temperature (250 to 350C) and high temperature (450 to 600C) operating conditions.
U.S. Pat. No. 5,451,558, xe2x80x9cProcess For the Reaction and Absorption of Gaseous Air Pollutants, Apparatus Therefor and Method of Making the Same,xe2x80x9d issued on Sep. 19, 1995 to L. Campbell et al., (xe2x80x9cthe Campbell et al.. Patentxe2x80x9d) discloses a catalytic material for the reduction of NOx from a turbine in a power generating stack, although the patent also refers at column 1, lines 13-14, generally to a process and apparatus for reducing pollutants xe2x80x9cwhich are produced by combustion of hydrocarbons or hydrogen in an engine or boiler, and primarily in a gas turbine.xe2x80x9d As disclosed at column 2, lines 23-37, the turbine exhaust gases are cooled to the range of 250 to 500F (about 121 to 260C) before contacting the catalytic/adsorbent material (column 2, lines 23-37) and the oxidation is stated (column 2, lines 45-48) to occur at temperatures in the range of 150 to about 425F (66 to 218C), most preferably in the range of 175 to 400F (about 79 to 204C). The catalytic species comprises an oxidation catalyst species which may comprise various metals including platinum group metals (see column 3, line 67 through column 4, line 3) deposited on a high surface area support which may be xe2x80x9cmade of alumina, zirconia, titania, silica or a combination of two or more of these oxides.xe2x80x9d The catalyst-containing high surface area support is coated with an adsorbent species which may comprise xe2x80x9cat least one alkali or alkaline earth compound, which can be a hydroxide compound, bicarbonate compound, or carbonate compound, or mixtures thereof.xe2x80x9d At column 3, lines 16-22, the xe2x80x9ccarbonate coatingxe2x80x9d is said to be a xe2x80x9clithium, sodium, potassium or calcium carbonate, and presently the preferred coating is a potassium carbonate.xe2x80x9d At column 4, lines 28-31, however, it is stated that the absorber comprises xe2x80x9cmost preferably sodium carbonate, potassium carbonate or calcium carbonate.xe2x80x9d The high surface area support containing the oxidation species and adsorbent may be coated onto xe2x80x9ca ceramic or metal matrix structurexe2x80x9d as a carrier (see column 4, lines 12-20). The catalytic material is applied to the carrier by coating the carrier with, e.g., platinum-impregnated alumina, and then wetting the alumina with an alkali or alkaline earth carbonate solution, and then drying the wetted alumina (see column 5, line 9 through column 6, line 12). The carriers may be alumina beads as illustrated in FIG. 1A, or a monolithic ceramic or stainless steel support as illustrated in FIG. 1C, both Figures being described at column 4, line 67, to column 5, line 8. The use of a metal monolith support for the catalytic/adsorbent material is suggested at column 5, lines 48-58. There is no suggestion in the Campbell et al. patent of criticality of, nor is any importance assigned to, the type of substrate or high surface area support to be used with a particular adsorbent species. In fact, as noted above, silica is one of four high surface area supports taught for use with compositions preferably including a potassium carbonate adsorbent.
The invention relates to a multi-zoned catalytic trap for conversion of NOx in an exhaust gas stream which emanates from an engine which is operated with periodic alternations between lean, and stoichiometric or rich, conditions. The multi-zoned catalytic trap comprising a first zone, a second zone and, optionally, one or more intermediate zones disposed between the first zone and the second zone. The fabrication of the catalytic trap in the form of such zones results in a trap which maintains high catalytic activity for NOx reduction at partial lean burn conditions, particularly at low temperature (250 to 350C) as well as high temperature (450 to 600C) operating conditions, without sacrificing catalytic activity in respect to removal of unburned hydrocarbons and carbon monoxide present in the exhaust gas stream. Moreover, the multi-zoned catalytic traps of the invention maintain their catalytic activity even after the traps are aged by exposure to high temperatures and lean operating conditions